* Astronomy

Baader Fringe Killer Filter 1.25"

Achromatic refractors, particularly short-tube telescopes with fast focal ratios suffer from noticeable chromatic aberration or false colour (seen as a blue/cyan fringing on high contrast objects).
The 1.25" Baader Planetarium Fringe Killer filter screws into your eyepiece, or more conveniently, your diagonal or Barlow lens, and simply removes the chromatic aberration.
The first noticeable thing is that it turns white objects slightly yellow, and that the blue sky is now slightly greenish blue. But when turned to a bright star, or object, the violet and yellow fringe usually surrounding it is gone (er, almost).

Baader Planetarium has taken minus violet filtration to a new level, the Fringe Killer brings a type of filtration, never before seen to the amateur. Rather than simply trimming the shorter wavelengths at a chosen wavelength and slope like all other minus violet filters, the Fringe Killer carefully attenuates a specific region of the spectrum.
This allowed Baader to tune the filter for maximum practical reduction of the most damaging defocused blue wavelengths, with only a faint yellow colour shift - without dimming and lowering image contrast by resorting to reduction of the important green-yellow wavelengths to crudely restore an apparent colour balance! (where the eye is very sensitive, and where refractors are their sharpest)
The result is a filter that removes the majority of colour fringe, without affecting image brightness all, imparting strong colouration, or reducing contrast at important wavelengths.

1.25":  ~ £40.00
2":  ~ £65.00

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This graph supplied by Baader shows that nearly all violet light (below 450nm) is blocked, with blue light (450 - 480nm) reduced to half. 
But, in real terms, this will not completely remove false colour from a fast achromatic refractor. The effect is similar to stopping down a 100mm fast refractor to 60mm or adding a yellow or light yellow (#8) filter.

So, Pricewise the benefit could be better (See Baader Contrast Booster Filter).
But it's a question of how annoying the false colour is on your refractor, and how much it degrades the detail - and how much you value your colour correction. 
I personally have a permanently fitted light pollution filter, that is used with a #8 or #11 filter that reduces the colour fringing down to acceptable levels.

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